Hadeshttps://hades.gsi.de
enHADES XXXV Collaboration Meetinghttps://hades.gsi.de/?q=node/305
<div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even" property="content:encoded"><p><a href="http://indico.gsi.de/event/6723/" target="_blank">HADES XXXV Collaboration Meeting, 19 - 23 February 2018, GSI Darmstadt</a></p>
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</div></div></div>Tue, 30 Jan 2018 15:01:41 +0000adminUser305 at https://hades.gsi.dehttps://hades.gsi.de/?q=node/305#commentsHADES hunts Dark Matterhttps://hades.gsi.de/?q=node/208
<div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even" property="content:encoded"><p>HADES has put an improved upper limit on the mixing of the photon with a hypothetical massive Dark Photon, the latter being the gauge boson mediating the interaction between Dark Matter particles.</p>
<h3>Dark Matter in the Universe</h3>
<p>The interpretation of current astrophysical observations results in the striking mass-energy budget of matter in the universe: ~75% Dark Energy, ~20% Dark Matter, and ~5% baryonic matter [1]. The latter number refers to stars and inter/intra-galactic gas, i.e. mainly free hydrogen. Dark Energy drives the presently observed accelerated expansion of the universe. It is homogeneously distributed and can be attributed to a cosmological constant or vacuum energy.<br />In extreme cases it may cause, in the future, such a sudden expansion that anything in the universe is disrupted – this would be the Big Rip. Dark Matter, in contrast, is bumpy and is needed to explain the formation of the ditribution of observed visible matter in the evolving universe, evidenced by the hierarchy of structures from (super)clusters of galaxies, galaxies, stars, planets and other compact objects such as meteorites, etc.</p>
<p>Many attempts have been made to pin down the nature of Dark Matter. Researchers believe that Dark Matter most likely comprised hitherto unknown particles which do not fit into the Standard Model of particle physics. The Standard Model is a theoretically sound quantum field theory with fundamental matter particles, such as quarks (bound in hadrons) and leptons (e.g. electrons and neutrinos), which interact via the exchange of force-carrier quanta, called gauge bosons (e.g., photons). Some of these species acquire their masses by the interaction with the Higgs boson. While evidence for the Higgs boson has been found recently at CERN (resulting in the 2013 Physics Nobel prize), the Standard Model looks now complete when supplemented by some neutrino masses. Indeed, nothing else seems to be needed to understand the wealth of atomic, subnuclear and particle physics phenomena – besides the remaining Dark Matter puzzle! This unsatisfactory state of affairs has therefore initiated worldwide efforts to search for Dark Matter candidates.</p>
<p>Among the list of candidates of Dark Matter is a hypothetical particle, often dubbed <em>U boson</em> or <em>Dark Photon</em>. These nicknames refer to the underlying theory construction: a second unitary (”U”) symmetry allows for quanta which are, in one respect, similar to photons – namely gauge bosons – but in another respect different from them: In fact, attributing to these quanta a mass makes them to Dark Photons having only a very weak interaction with normal matter. Through its mixing with the normal photon, the Dark Photon can also decay into detectable electron-positron pairs. One arrives hence at a scenario where any electromagnetic process involving ”ordinary” virtual photons might be affected by the Dark Photon visible as (small) deviations from the expected Standad Model behavior. In particular, the recently observed discrepancy of the muon magnetic moment anomaly g<sub>μ</sub> − 2 has been discussed as a possible sign of a Dark Photon.</p>
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<h3>The HADES Dark Photon search</h3>
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<p><img alt="" src="http://www-hades.gsi.de/sites/default/files/web/media/documents/dark_matter_article/HADES_dark_matter.png" style="width: 450px; height: 384px;" /></p>
<p>Figure 1: The 90% CL upper limit on ε<sup>2</sup> versus the U-boson mass obtained<br />from the combined analysis of HADES data (solid black curve).</p>
<p>We have searched for a narrow U -&gt; e−e+ decay signal in dielectron spectra obtained with HADES in 3.5 GeV p+p and p+Nb reactions, as well as in the 1.756 GeV/u Ar+KCl reaction [2]. In contrast to previous experiments focussing on a specific decay channel, our analysis is based on the inclusive measurement of all e+e− pairs produced in a given mass range, i.e. from Dalitz decays of the π<sup>0</sup>, η, and Δ mostly. Using a maximum-likelihood method we have extracted an upper limit (UL) at a confidence level CL = 90%. With known detector efficiencies and decay branching fractions, this UL has then been transformed into an UL on the mixing parameter ε<sup>2</sup> as shown in Fig. 1 together with limits from the searches conducted by BaBar, KLOE-2, APEX, WASA at COSY, and A1 at MAMI. In particular at low masses (M<sub>U</sub> &lt; 0.1 GeV/c<sup>2</sup>) we have improved on the recent result obtained by WASA, excluding now to a large degree the parameter range allowed by the muon g − 2 anomaly. At higher masses, the sensitivity of our search is compatible with, albeit somewhat lower than the KLOE-2 analysis of Φ decays.</p>
<h3>References</h3>
<p>[1] J. Behringer et al.. (Particle Data Group), Phys. Rev. D 86 (2012) 010001.<br />[2] G. Agakishiev et al. Phys. Lett. B 731 (2014) 265.</p>
</div></div></div>Tue, 08 Apr 2014 12:11:38 +0000adminUser208 at https://hades.gsi.dehttps://hades.gsi.de/?q=node/208#commentsPDG entries - HADES aims at precisionhttps://hades.gsi.de/?q=node/41
<div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even" property="content:encoded"><p>The HADES experiment has been designed to study di-electron emission in heavy ion reactions. However, its powerful particle identification capabilities and excellent momentum resolution also allows precise measurements of hadron properties in elementary collisions. Below we present two recent results, which are cited in the Particle Data Group (PDG 2012).</p>
<h3>Rare η→e<sup>+</sup>e<sup>-</sup> decays</h3>
<p>Experimental searches for the rare decays of the η meson are essential for testing the validity of the Standard Model (SM). Among other decay channels the direct decay of the η meson intoan electron-positron pair is of particular interest. In the SM the respective decay is mediated by two virtual photons with an internal electron line which is suppressed by helicity conservation.</p>
<p><img alt="" src="http://hades-new.gsi.de/sites/default/files/web/media/documents/pdg_article/Fig1_eta_ep%2Bem.png" style="width: 400px; height: 274px;" /></p>
<p>Fig. 1 Expected signal corresponding to the η → e<sup>+</sup>e<sup>-</sup> decay with a branching ratio of 4.9 × 10<sup>-6</sup> superimposed on e+e- invariant mass distribution measured by HADES in proton-proton collisions at 3.5 GeV.</p>
<p>The branching ratio (BR) for such a process can be estimated from the known η→γγ decay and leads to a lower limit of BR(η → e<sup>+</sup>e<sup>-</sup>) ≥ (1.77 ± 0.009) × 10<sup>-9</sup>. A larger value, (2.4 ± 0.33) × 10<sup>-9</sup>, can be derived from the observed η → μ<sup>+</sup>μ<sup>-</sup> channel assuming that the ratio between the real and the imaginary parts of the decay amplitude is the same for both decays. On the other hand, the experimental upper limit for the η → e<sup>+</sup>e<sup>-</sup> branching ratio is 2.7×10<sup>-5</sup> [1], i.e.four orders of magnitude larger then theory suggests. Thus, experiment at the moment cannot exclude contributions to the decay amplitude from processes beyond the SM. For example, it has been suggested that direct decays of pseudo-scalar meson into electron-positron pairs can be enhanced by a coupling to a new undiscovered vector meson responsible for the annihilation of a neutral light dark matter particle [2]. Such searches become of large interest because of the enhanced π<sup>0</sup> → e<sup>+</sup>e<sup>-</sup> transition rate reported by the KTeV collaboration [3]. Recently, the HADES collaboration reported dielectron measurements in proton proton interactions at 3.5 GeV kinetic beam energy [4]. For the first time at this beam energy the inclusive production cross sections of π<sup>0</sup>, η, ρ and ω mesons have been measured. From the smooth spectral distribution of electron positron pairs in the η mass region an improved upper bound for the η → e<sup>+</sup>e<sup>-</sup> decay could be deduced lowering the old value by a factor of about 6 [4]. The new value of BR = 5.6 ×10<sup>-6</sup> will be quoted in the 2012 edition of the PDG as the best estimated upper limit for the η → e<sup>+</sup>e<sup><sub>–</sub></sup> decay.</p>
<h3>Λ and Σ Hyperons in p+p Collisions</h3>
<p>The measurement of Λ, Σ hyperons below the K-N threshold is of particular interest for understanding Kaon-Nucleon interaction in different environments.For example, widely distputed questions like of existence of kaonic clusters (bound states of K<sup>-</sup> and nucleons) or the lowering of the K<sup>-</sup> mass in dense nuclear matter are strongly related to the understanding of the holy grail of Kaon-Nucleon interaction: the Λ(1405). However, about the production of the lambda hyperon in N-N collisions very little is known [5] . Indeed, to separate it from the overlapping Σ(1385), which has a comparable width of 40-50 MeV and decays into similar channels, is very difficult. HADES has recently measured the production of both hyperons in proton-proton collisions at a kinetic beam energy of 3.5 GeV [6,7]</p>
<p><img alt="" src="http://hades-new.gsi.de/sites/default/files/web/media/documents/pdg_article/Fig2_sigma1385.png" style="width: 400px; height: 287px;" /></p>
<p>Thanks to the excellent momentum resolution of HADES and capability for reconstruction of weak decays through their secondary decay vertices, both resonances could be identified and separated from other, much more abundant reaction channels. In particular the Σ(1385)<sup>+</sup> decay into the Λ(1115) and π<sup>+</sup> was analyzed and allowed to extract the Σ<sup>+</sup> spectral function with high precision, including the evaluation of the systematic errors, the latter not being available for older measurements reported in the PDG so far.</p>
<p>[1] WASA collaboration, M. Berłowski et al., Phys. Rev. D77(2008) 032004<br />[2] Y. Kahn, M. Schmitt, and T. M. P. Tait, Phys. Rev. D78 (2008) 115002, arXiv:0712.0007 [hep-ph].<br />[3] KTeV Collaboration, E. Abouzaid et al. Phys. Rev. D75 (2007) 012004<br />[4] L.S. Geng E. Oset, EPJA34 (2007) 405<br />[5] HADES Collaboration, Eur. Phys. J. A48, 64 (2012).<br />[6] HADES Collaboration, Phys. Rev. C 85, 035203 (2012)<br />[7] L. Fabbietti, E. Epple et. al (HADES Coll). arXiv:1202.023</p>
</div></div></div>Fri, 22 Jun 2012 11:21:26 +0000adminUser41 at https://hades.gsi.dehttps://hades.gsi.de/?q=node/41#comments